1. Field of the Invention
The present invention relates to an exhaust gas recirculation control system for an internal combustion engine.
2. Description of the Prior Art
In order to control an exhaust gas recirculation system so as to obtain an optimum exhaust gas recirculation rate according to an engine operating condition, one means as shown in FIG. 1 has been suggested.
In FIG. 1, an engine 1 comprises an intake duct 2 and an exhaust duct 3, these ducts being connected by an exhaust gas recirculation passage 5 for recirculating part of an exhaust gas downstream of an intake throttle valve 4.
For the purpose of controlling the exhaust gas recirculation rate depending upon the engine operating condition, a control valve 6 is provided in the recirculation passage 5. Control valve 6 is adapted to be controlled by negative operating pressures which are adjusted by means of a solenoid valve 8 adapted to be responsive to signals from a control circuit 7 including a micro computer.
A part of the intake negative pressure downstream of the intake throttle valve 4 is introduced into a constant pressure valve 10 to produce therein a constant negative pressure. The constant negative pressure is mixed with the air or atmosphere introduced in the solenoid valve 8 which receives the air depending upon its opening degree to produce a control negative pressure which is supplied to a diaphragm drive 9 of the control valve 6. As the negative degree of the control negative pressure increases, the opening degree of the control valve 6 increases and therefore the exhaust gas recirculation flow rate also increases.
The control circuit 7 includes an interface, a microprocessor, a read only memory (ROM) and a random access memory (RAM). Signals representative of operating conditions of the engine from, for example, an engine revolution sensor 11, an intake air flow sensor 12 or a throttle valve open degree switch 13 are supplied to the control circuit 7 which selects an exhaust gas recirculation rate previously set corresponding to the engine operating condition on the basis of the received signals to produce a pulse signal which is supplied to the solenoid valve 8 to obtain a desired recirculation rate.
With this system, therefore, the exhaust gas recirculation can be controlled with a high accuracy to an extent such that both a driving performance and exhaust gas condition (NOx reduction effect) required for the engine are brought under satisfactory conditions.
When first installed this system operates with a high degree of accuracy. However, it unavoidably encounters the problem of changes due to aging. The accuracies of the solenoid valve 8 and constant pressure valve 10 become lower as time passes which cause delicate errors in controlling the exhaust gas recirculation which make it difficult to maintain the stable control performance.